Large-grain Polycrystalline Silicon Research Articles

Solid phase epitaxial thickening of boron and phosphorus doped polycrystalline silicon thin films formed by aluminium induced crystallization technique on glass substrate

Aluminium induced crystallization (AIC) technique can be used to form the high-quality and large-grained polycrystalline silicon (poly-Si) thin films, which are with the thickness of ~200 nm and used as a seed layer, on silicon nitride coated glass substrate. Thanks to aluminium metal in AIC process, the natural doping of AIC thin films is p+ type (~2 × 1018 cm−3). On the other hand, recombination of carriers can be controlled by partial doping through the defects that may have advantages to improve the thin film quality by the overdoping induced passivation. In this study, boron (B) and phosphorus (P) doped AIC seed layers were thicken to ~2 μm by solid phase epitaxy (SPE) technique at 800 °C for 3 h under nitrogen flow in a tube furnace. During the crystallization annealing, exodiffusion of dopants was formed through the SPE film from the AIC seed layer. Optical microscope and electron back scattering diffraction technique (EBSD) were used to analyse the structural quality of the Si films. The poly-Si layer with an average grain size value of ~32 μm was formed by AIC + SPE technique for P doped samples while EBSD analysis gave no results for B doped samples due to the quite deterioration on the surface of the films. AIC + SPE films were analyzed in terms of structural properties by using micro-Raman Spectroscopy and X-ray diffraction systems. The results showed that the crystallinity of compressive stress formed AIC + SPE films reached up to 98.55%. Additionally, the Raman analysis pointed out that no temperature-induced stress were generated in the AIC + SPE films while compressive stress was induced by increasing the annealing duration for doped AIC film. For all samples, the preferred orientation was 〈100〉, and the crystallite size up to 44.4 nm was formed by phosphorus doping of AIC films. The doping efficiency was determined by time-of-flight secondary ion mass spectroscopy for doped samples. A graded n+n doping profile was obtained by exo-diffusion of phosphorus from the overdoped seed layer during the epitaxial thickening while boron doping of SPE film has failed with exo-diffusion of boron from AIC seed layer into SPE film. Finally, high-quality n+n type poly-Si films were fabricated on glass substrate by using AIC + SPE technique.

Rapid vapour deposition and in situ melt crystallization for 1 min fabrication of 10 μm-thick crystalline silicon films with a lateral grain size of over 100 μm

We developed a film deposition method which yielded continuous polycrystalline Si films with large lateral grain sizes of over 100 μm and thicknesses of ∼10 μm in 1 min on growth substrates other than silicon wafers in a single-step process. The silicon source is heated to ∼2000 °C, much higher than the melting point of Si, which enables a high deposition rate. Controlling the temperature of the growth substrate, initially above and later below the melting point of Si, allows the seamless lateral to vertical growth of crystalline silicon grains. Thermally and chemically stable substrates of quartz glass and alumina with a 0.1 μm-thick amorphous carbon layer were effective; liquid silicon wetted well by forming a thin SiC interlayer while substrates stayed stable. Such large-grain polycrystalline silicon films synthesized rapidly in 1 min may be used for low-cost, stable and flexible thin film photovoltaic cells.

Photoluminescence study of polycrystalline silicon thin films prepared by liquid and solid phase crystallization

Polycrystalline silicon thin films were prepared by depositing amorphous or microcrystalline silicon layers onto glass substrates and subsequent crystallization via solid or liquid phase crystallization approaches. Differences in layer morphology and quality were characterized using low temperature photoluminescence (PL) spectroscopy and electron backscatter diffraction spectroscopy. The evaluation of spectral and spatially resolved PL response was used to conclude about the relative density of defects in these layers. Liquid phase crystallization using a line shaped electron beam proves to be the most promising technique in order to obtain large-grained high quality polycrystalline silicon thin films.

Fabrication of Large-Grain Thick Polycrystalline Silicon Thin Films via Aluminum-Induced Crystallization for Application in Solar Cells

The fabrication of large-grain 1.25 μm thick polycrystalline silicon (poly-Si) films via two-stage aluminum-induced crystallization (AIC) for application in thin-film solar cells is reported. The induced 250 nm thick poly-Si film in the first stage is used as the seed layer for the crystallization of a 1 μm thick amorphous silicon (a-Si) film in the second stage. The annealing temperatures in the two stages are both 500°C. The effect of annealing time (15, 30, 60, and 120 minutes) in the second stage on the crystallization of a-Si film is investigated using X-ray diffraction (XRD), scanning electron microscopy, and Raman spectroscopy. XRD and Raman results confirm that the induced poly-Si films are induced by the proposed process.

Fabrication of large-grained thin polycrystalline silicon films on foreign substrates by titanium-assisted metal-induced layer exchange

Metal-induced layer exchange (MILE) is a well-known method to grow large-grained high quality polycrystalline silicon on foreign substrates. We have modified the commonly used layer stack by an additional titanium interfacial layer (substrate/metal/titanium/oxide/amorphous silicon). The resulting layer exchange process is called titanium-assisted metal-induced layer exchange (Ti.MILE). For the investigated metals, Al (Ti.ALILE) and Ag (Ti.AgILE), the additional Ti layer does not affect the overall layer exchange process but results in a strong enlargement of the grains in the resulting polycrystalline silicon layer up to 250 μm. We have investigated the influence of the titanium interfacial layer on the process dynamics and grain growth. Furthermore, the structural and optical properties of the resulting polycrystalline silicon layer are investigated by means of different analysis methods.

Large-Grain Polycrystalline Silicon Films Formed through Flash-Lamp-Induced Explosive Crystallization

Large-Grain Polycrystalline Silicon Films Formed through Flash-Lamp-Induced Explosive Crystallization

Large-Grain Polycrystalline Silicon Films Formed through Flash-Lamp-Induced Explosive Crystallization

The flash lamp annealing (FLA) of electron-beam- (EB-) evaporated amorphous silicon (a-Si) films results in the formation of polycrystalline Si (poly-Si) films with at least a few µm long grains stretching along lateral crystallization directions. Unlike the case of using chemical-vapor-deposited (CVD) hydrogenated a-Si films as precursors, no peeling of Si films occurs even in the absence of Cr adhesion layers. Such a flash-lamp-induced crystallization occurs also in doped EB-evaporated a-Si films as in the case of undoped films. The p+/p-/n+ stacked structure is sufficiently kept even after crystallization, although the profiles of dopants are slightly modified. This fact clearly indicates that the crystallization observed is not based on liquid-phase epitaxy (LPE) after the complete melting of the whole a-Si precursor during millisecond-order treatment but through LPE-based explosive crystallization (EC), self-catalytic lateral crystallization driven by the release of latent heat. The formation of poly-Si films with large grains and the sufficient preservation of dopant profiles would lead to the utilization of the poly-Si films formed for solar cell devices.

Solid-phase crystallization of ultra high growth rate amorphous silicon films

In this paper, we report on the deposition of amorphous silicon (a-Si:H) films at ultra-high growth rate (11–60 nm/s) by means of the expanding thermal plasma technique, followed by solid-phase crystallization (SPC). Large-grain (∼1.5 μm) polycrystalline silicon was obtained after SPC of high growth rate (∼25 nm/s) deposited a-Si:H films. The obtained results are discussed by taking into account the impact of the a-Si:H microstructure parameter R* as well as of its morphology, on the final grain size development.

Large-Grain Polycrystalline Silicon Solar Cell on Epitaxial Thickening of AIC Seed Layer by Hot Wire CVD

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Large-grain polycrystalline silicon (poly-Si) films were prepared on foreign substrates by the epitaxial thickening of seed layers. The seed layers were formed by aluminum-induced crystallization (AIC). Large-grain n-i-p poly-Si solar cells were deposited on epitaxial seeds by hot-wire chemical vapor deposition (HWCVD). Highly (93%) crystalline fractions with a lateral grain size of 5 <formula formulatype="inline"><tex Notation="TeX">$\mu\hbox{m}$</tex></formula> and an intrinsic layer were grown without incubation. These techniques were employed to prepare large-grain poly-Si thin-film solar cells. An ITO/n-i-p (HWCVD)/p<formula formulatype="inline"><tex Notation="TeX">$+$</tex></formula> (AIC)/Ti/glass-structured poly-Si thin-film solar cell with an initial efficiency of 5.6% was obtained. </para>

Gettering in Large-Grained Thin Polycrystalline Silicon Films on Glass Substrate

Large-grained polycrystalline silicon (poly-Si) films on a glass substrate are good candidate materials for fabricating poly-Si thin-film transistors (TFTs) and poly-Si solar cells. The gettering of metal impurities in thin poly-Si films is one of the key techniques for realizing high-performance, high-reliability, and high-efficiency TFTs and solar cells. In this study, the gettering of large-grained thin poly-Si films with a thickness of 100 nm containing Ni impurities was studied. A large-grained Ni-doped thin poly-Si film was fabricated on a glass substrate by Ni-induced solid phase crystallization (SPC) followed by cw green laser recrystallization. The grain size of the poly-Si film was 1 ×10 µm2. The behavior of Ni impurities in the large-grained thin poly-Si film was evaluated by scanning transmission electron microscopy (STEM), STEM energy-dispersive X-ray spectroscopy (STEM–EDX), and electron diffraction (ED) analysis. It was observed that Ni was stabilized through the formation of nickel-disilicide (NiSi2) at the triple junction during low-temperature device fabrication.

Nickel-induced crystallization of amorphous silicon

The nickel-induced crystallization of hydrogenated amorphous silicon (a-Si:H) is used to obtain large grained polycrystalline silicon thin films on glass substrates. a-Si:H is deposited by plasma enhanced chemical vapour deposition at 200 °C, preparing intrinsic and slightly p-doped samples. Each sample was divided in several pieces, over which increasing Ni concentrations were sputtered. Two crystallization methods are compared, conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The crystallization was followed by optical microscopy and scanning electron microscopy observations, X-ray diffraction, and reflectance measurements in the UV region. The large grain sizes obtained – larger than 100 μm for the samples crystallized by CFA – are very encouraging for the preparation of low-cost thin film polycrystalline silicon solar cells.

Fabrication of large-grain polycrystalline silicon for solar cells

The rapid recrystallization of a-Si films utilizing excimer laser crystallization (ELC) is investigated. The melt duration of liquid silicon (liquid Si) is measured by in-situ time-resolved optical measurements during ELC. The grain size of crystallized poly-Si films are characterized by FE-SEM. The substrate temperature as a function of the longest melt duration of liquid Si for different Si film thicknesses are presented. In-situ timeresolved optical measurements reveal that the longest melt duration of 400-nm-thick a-Si thin films at a substrate temperature of 500 °C is 1714 ns. The diameter of the disk grain as large as 4.7 μm is produced by ELC, which can be applied to solar cells with a high conversion efficiency and reliability.

Hot-Wire Chemical Vapor Deposition Epitaxy on Polycrystalline Silicon Seeds on Glass

AbstractDuring the last few years, hot-wire chemical vapor deposition (HWCVD) has been explored as a low-temperature process for epitaxially thickening c-Si seeds layers on low cost substrates. Here, we demonstrate HWCVD epitaxy on thin polycrystalline silicon seed layers on borosilicate glass substrates. The crystal Si seeds are large-grained (~10 µm) polycrystalline silicon that were fabricated by Al-induced crystallization of a-Si. We report the growth of 0.5 µm of epitaxy at ~670°C.

Large-grained polycrystalline silicon films on glass by argon-assisted ECRCVD epitaxial thickening of seed layers

This work reports on the low temperature ( ⩽ 550 ∘ C ) epitaxial growth of Si films on polycrystalline seed layers made by aluminium-induced crystallisation (AIC) on glass. The seed layers were chemically mechanically polished before use and electron–cyclotron resonance chemical vapour deposition (ECRCVD) was used to deposit the Si films. The quality of the deposited films depends strongly on the addition of Ar to the hydrogen rich silane growth environment. With Ar, epitaxial growth has been achieved to a thickness of ∼ 2.7 μ m . Electron backscatter diffraction (EBSD) analysis shows a similar grain structure to the starting seed layers with a preferred (1 0 0) orientation and grain sizes approaching ∼ 10 μ m for samples up to ∼ 1.7 μ m thick. Thicker films ( 2.7 μ m ) show a more mixed orientation with an increase in the density of smaller grains. Nevertheless, more than 95% of the film surface can be indexed suggesting large grained crystal growth has occurred on all grain orientations present in the seed layer. Similar behaviour is also exhibited by the films grown without Ar but these films are more defective and the deterioration in the EBSD orientation maps occurs at a significantly lower thickness. The role of Ar in the growth process is briefly discussed.

Localisation of the p-n junction in poly-silicon thin-film diodes on glass by high-resolution cross-sectional electron-beam induced current imaging

High-resolution scanning electron microscope, focused ion beam (FIB) microscope, and electron-beam induced current (EBIC) images are taken on large-grained polycrystalline silicon thin-film diodes on glass. To aid understanding of the plan-view EBIC images obtained, trenches are cut into the diodes in the FIB microscope, and the p-n junction then located and examined in the diode's cross section using high-resolution EBIC. Diffusion of aluminium atoms from the heavily doped emitter region into the base region of the diode along grain boundaries during material fabrication is found to have locally altered the junction's location and therefore alters the EBIC signal in these regions. Using this method allows a fast and easy location of the p-n junction in these diodes, in addition to aiding in the identification of unusual junction deviations which complicate the interpretation of plan-view EBIC images.

Large-grain poly-crystalline silicon thin films prepared by aluminum-induced crystallization of sputter-deposited hydrogenated amorphous silicon

A metal-induced crystallization (MIC) technique was used to produce large-grain poly-crystalline silicon. Two sets of samples were prepared by first sputtering Al onto glass substrates. For one set of samples, hydrogenated amorphous silicon (a-Si:H) was sputtered on top of the Al without breaking the vacuum. For the second set, the samples were taken out of the vacuum chamber and exposed to the atmosphere to grow a very thin layer of native aluminum oxide before sputter depositing the a-Si:H. Both sets of samples were then annealed at temperatures between 400 and 525 °C for 40 min. X-ray diffraction patterns confirmed the crystallization of the samples. Scanning Auger microanalysis was used to confirm that the a-Si:H and Al layers exchanged positions in this structure during the crystallization process. Auger mapping revealed the formation of large grain poly-silicon (10–20 μm). A model is proposed to explain how the crystallization process progresses with anneal temperature.

Large-grained polycrystalline silicon on glass for thin-film solar cells

We have investigated the formation of large-grained polycrystalline silicon (poly-Si) films on glass for thin-film solar cells using the ‘seed layer concept’ which is based on the epitaxial thickening of a thin large-grained poly-Si template (seed layer). Due to the glass substrate all process steps are limited to a temperature of about 600 °C. The aluminium-induced layer exchange (ALILE) process based on the aluminium-induced crystallisation (AIC) of amorphous Si has been used to prepare p +-type seed layers featuring large grains and a high preferential (100) orientation of the surface. The seed layers have been thickened by electron cyclotron resonance chemical vapour deposition (ECRCVD) to form the p-type absorber of the solar cell. First poly-Si thin-film solar cell structures have been prepared by deposition of an n +-type a-Si : H emitter. So far an open circuit voltage of 284 mV has been reached without any additional treatments like defect annealing and defect passivation.

Large-grain polycrystalline silicon film by sequential lateral solidification on a plastic substrate

A large-grain polycrystalline silicon film was obtained on a plastic substrate by sequential lateral solidification. With various combinations of sputtering powers and Ar working gas pressures, the conditions for producing dense amorphous silicon (a-Si) and SiO2 films were optimized. The successful crystallization of the a-Si film is attributed to the production of a dense a-Si film that has low argon content and can endure high-intensity laser irradiation.

Direct fabrication of large-grain polycrystalline silicon thin films by RF-biased RF-PECVD at low temperature

Polycrystalline silicon (Poly-Si) films with large grains are successfully fabricated on glasses in a self-assembled RF-biased RF-PECVD tubular system. Both inductively coupled and capacitively coupled RF powers are employed. High purity (99.99%) SiH 4, H 2 and Ar are introduced as resource gases. All crystalline films obtained in this study exhibit (111)-Si preferred orientation. Poly-Si films with large grains (about 0.15–0.3 mm in size) which are aggregate of fine crystals (about 50 nm in size) are deposited at the deposition temperature less than 77 °C. However, the improvement of crystallinity and the grain size are still under investigation.

An Infrared Sensing Material Using a Large-Grain Polysilicon Film

We first report the feasibility of using large-grain polycrystalline silicon films as a sensing material for infrared bolometers. To increase the average grain size of polysilicon films, we used seed selection through ion channeling, which resulted in a large grain size of 1670 Å. The temperature coefficient of resistance (TCR) at 20°C and the grain boundary defect density of the film were as high as -2.46%/K for a resistivity of 30 Ω cm and about 1.752×1012/cm2, respectively. From the measurement of noise characteristics of the film, the value of k, 1/ f noise parameter, was calculated to be 1.35×10-9. As a result, the estimated detectivity was found to approach 5.6×108 cm Hz1/2/W.